GREAT BARRIER REEF
// Outlook Report 2014
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3.3 Current condition and trends of chemical processes
3.3.1 Nutrient cycling
Nutrient cycling plays a critical role in maintaining ecosystem health. Most nutrient concentrations (for example nitrogen and phosphorus) in the open ocean are low.95 Low concentrations of nitrates, in particular, severely limit plant productivity. Coral reefs farther naturally from land are able to survive in low nutrient waters by having a high level of nutrient cycling.95 For reefs nearer land, additional nutrients are derived naturally from terrestrial sources. An overabundance of nutrients increases plant growth, resulting in effects like algal blooms and increased macroalgal growth, which can affect ecosystem health96, for example through reducing available light for seafloor communities and trapping sediment. Modelling of pre-European exposure to dissolved inorganic nitrogen from river discharges suggests that it was concentrated very close to the coast around river mouths, with the largest plume adjacent to the Wet Tropics rivers and the Burdekin River.34 Since European settlement in the adjacent catchment, nutrient loads entering the Region are estimated to have increased40,97 almost two-fold for both nitrogen and phosphorus.97 Most inshore areas of the southern two-thirds of the Region are now exposed to nutrients at elevated concentrations98 (Figure 3.9), disrupting nutrient cycling in the ecosystem. Recent investments in improving land management practices from 2009 to 2013 have resulted in a modelled 16 per cent reduction in the average annual dissolved inorganic nitrogen load leaving the catchment.56,99 Long-term benefits are expected to follow for the Region’s ecosystem.98,100 However, the lag between improved practices and environmental benefits is likely to mean that the nutrient cycle will continue to be affected for some decades.57 Offshore and remote northern areas of the Region are believed to be mostly unaffected by increased nutrients and hence nutrient cycling is assumed to be functioning naturally.
Most inshore areas of the southern two-thirds of the Region are exposed to elevated nutrient concentrations.
3.3.2 Ocean pH
It is estimated that an increase in the amount of carbon dioxide absorbed by the ocean has already caused a decrease in global ocean acidity of 0.1 pH units compared to the long-term average.101,102 From a current pH of 8.1102, it is predicted that the ocean could fall to a pH of about 7.6 by 2100, with slight regional variation.66
Figure 3.9 exposure to dissolved inorganic nitrogen, 2007–2011
Nutrients, such as dissolved inorganic nitrogen, are now present in the ecosystem at far higher concentrations than those likely to have been present prior to European settlement. The assessment classes (high, moderate and low) are relative and derived from a combination of scaled river loads data and flood plume frequency analysis from remote sensing data. The mean of the five annual distributions was selected as a way of factoring in inter-annual variability in river discharge, although it is recognised that this period was characterised by several extreme rainfall events.
Source: Brodie et al. 2013 60
The pH of the ocean is of vital importance to many marine animals and plants. Decreases in ocean pH can have a range of impacts on species and habitats (see Section 6.3.2) and it is predicted that the ecosystem will be affected on a Reef-wide scale. For example, more acidic water can reduce the ability of some animals to grow strong calcium carbonate shells or skeletons101,103 (Section 3.4.8). The consequences of decreases in pH and other changes in ocean chemistry are just beginning to be understood.102
Decreasing ocean pH is likely to affect the ecosystem Reef-wide in the future.
3.3.3 Ocean salinity